The HSE is in the process of contacting thousands of patients to reschedule appointments following Friday's cyber attack on its networks.
All outpatient and radiology services are impacted, however, chemotherapy and dialysis services are continuing as normal.
COVID-19 vaccination appointments are going ahead as normal but there are delays to the testing and tracing programme.
Meanwhile, efforts are continuing the identify the extent of the attack, and if personal patient data has been stolen.
The health service said it is prioritising emergency services over the coming days, with staff told to “protect unscheduled and urgent care”.
Anne O'Connor, Chief Operations Officer at the HSE, told On The Record with Gavan Reilly that work has begun on rebuilding IT systems but the process will be slow.
"It's been an extraordinary few days for us, the reality is there's not much back up and running," she said.
"Where we're at at the minute is we have between yesterday morning and this morning found that we do have some clean back-up data available to rebuild our servers from.
"However, we have thousands upon thousands of virtual servers so each server is going to have to be rebuilt and brought back up individually.
"So it's going to be a slow process, our system has been very significantly compromised across the board, both in terms of our patient management systems and also our other systems which, whilst they're lower risk from a patient perspective, in terms of even just communicating with patients, with GPs, communication between hospitals and community services, that's all gone too."
Work is under way to assess how much patient data was potentially compromised in the attack, she added, with teams both domestically and internationally working to ascertain the impact on data or what has been taken.
"The nature of this attack is that our data has been encrypted and possibly stolen, but we just don't know that bit of it yet," Ms O'Connor said.
"[The perpetrators] took the data and encrypted it, our data is encrypted, but they encrypt it and seek money to give us to code to release it back to us.
"From our perspective, we're looking at rebuilding using clean backup data."
Ms O'Connor said her focus is on getting the patient management systems and radiology systems back online again.
"All of our diagnostic capability in terms of radiology has gone, we are currently disconnecting machines after we have spent years developing an integrated system," she explained.
"The risk is, for example, if you have somebody coming into a hospital for anything, we have no capability to look back at previous tests or scans, we can't order lab tests or radiology electronically.
"So for anyone coming in, it's back to manual, handwritten notes."
People with outpatient appointments are advised to monitor the HSE website to check what services and hospitals have been impacted.